Method for fabricating a semiconductor devices provided with low melting point metal bumps

ABSTRACT

A semiconductor device ( 1 ) comprising electrodes formed on a semiconductor chip ( 2 ) and bumps ( 3 ) which consist of a low melting point metal ball spherically formed and having a given size and which are adhesive bonded to the electrodes ( 5 ). The electrodes ( 5 ) are formed from an electrode material of Cu or a Cu alloy, Al or an Al alloy, or Au or a Au alloy. When the electrode material is composed of Al or an Al alloy, the electrodes contain, on the electrode material layer of Al or an Al alloy, at least one layer ( 6 ) composed of a metal or metal alloy (preferably a metal selected form Ti, W, Ni, Cr, Au, Pd, Cu, Pt, Ag, Sn or Pb, or an alloy of these metals) having a melting point higher than the electrode material. The low melting point metal balls ( 3 ) are adhesive bonded to the electrodes ( 5 ) preferably with a flux. The low melting point metal balls ( 3 ) adhesive bonded to the respective electrodes ( 3 ) may also be reflowed to form semispherical bumps ( 10 ) before use.

This application is a divisional application under 37 C.F.R. §1.53(b) ofprior application Ser. No. 09/254,119 filed Apr. 16, 1999 which is a 35U.S.C. 371 of PCT/JP97/02987 filed Aug. 27, 1997. The disclosures of thespecification, claims, drawings and abstract of application Ser. No.09/254,119 and PCT/JP97/02987 are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device provided withlow melting point metal bumps and a process for producing the same.

BACKGROUND ART

Semiconductor devices have currently been used widely in various fields.The semiconductor devices are usually used by mounting them onsubstrates. The mounting methods include bonding methods such as tapeautomated bonding (TAB), wire bonding and flip chip bonding.

The TAB and wire bonding are technologies by which a semiconductordevice is mounted on a substrate through leads. The leads are arrangedin one row per peripheral side of the semiconductor device. Thetechnologies are, therefore, not suited to high density mounting of thesemiconductor devices. In contrast to the technologies mentioned above,flip chip bonding is a technology by which the electrodes of asemiconductor device are directly-connected to the electrode terminalson the substrate through a bonding metal. Since the electrodes of thesemiconductor device can be provided in a lattice-like form on theentire surface, the technology is suited to high density mounting.Various solders are generally used as bonding metal in flip chip bondingbecause the bonding is conducted by melting at low temperature.

In flip chip bonding, semiconductor devices provided with low meltingpoint metal bumps for bonding placed on electrodes are used, and thesemiconductor devices are connected to the electrode terminals ofsubstrates by a reflowing procedure by which the bumps are melted andsolidified again.

In general, the bumps are formed by vapor deposition or plating.However, such bump formation methods must all repeat complicatedtreatments steps using a mask. Moreover, in the method of forming thebumps by vapor deposition, a bump material is deposited on portionswhere the bumps are not to be formed, and the deposition amount thereonis very large. The method is, therefore, not a preferred one in view ofthe cost and efficiency. Moreover, wet plating such as electroplating orelectroless plating fouls wafers and causes an environmental problem,and countermeasures against such problems are indispensable. Asillustrated above, conventional methods for forming the bumps arerelatively costly, and practical use of the methods is restricted.

There is a stud bump procedure as a method for forming bumps other thanvapor deposition and plating. Since bumps are formed one by one in theprocedure, the production efficiency is low, and in addition the bumpamount tends to vary among the bumps. Accordingly, securing uniformityin bonding the semiconductor devices and the substrates is difficult.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a semiconductor deviceprovided with low melting point metal bumps of high quality and capableof being mounted on a substrate by flip chip bonding, and a process forproducing the same.

The semiconductor device of the present invention comprises electrodesformed on a semiconductor chip, and is provided with bumps eachconsisting of a low melting point metal ball which is spherically formedand has a given size, and adhesive bonded to the electrodes.

The low melting point metal balls are preferably adhesive bonded to therespective electrodes with a flux.

The electrodes on the semiconductor chip are preferably formed from anelectrode material of Cu or a Cu alloy, Al or an Al alloy, or Au or a Aualloy.

When the electrode material is Al or an Al alloy, at least one layer ofa metal or a metal alloy having a melting point higher than theelectrode material is preferably laminated to the layer formed from theelectrode material.

The laminated layers are preferably formed from a metal selected fromTi, W, Ni, Cr, Au, Pd, Cu, Pt, Ag, Sn and Pb, or an alloy of thesemetals.

It is preferred that, of the layers laminated to the electrode materiallayer, the layer contacted with the electrode material layer be formedfrom Ti, W, Ni, Cr, Pd, Cu or Pt, or an alloy of these metals, and thatthe layer contacted with the low melting point metal ball be formed fromNi, Au, Pd, Cu, Pt, Ag, Sn or Pb, or an alloy of these metals.

The process for producing a semiconductor device according to thepresent invention is a process for producing a semiconductor devicehaving electrodes formed on a semiconductor chip, and provided withbumps which consist of low melting point metal balls each being formedspherically and having a given size, and which are adhesive bonded tothe respective electrodes, and is characterized by the low melting pointmetal balls being adhesive bonded and fixed to the respective electrodeswith a flux.

The flux is preferably applied to the electrodes.

On another aspect of the present invention, the process for producing asemiconductor device according to the present invention is a process forproducing a semiconductor device provided with low melting point metalbumps on the electrodes on a semiconductor chip, the process comprisingthe steps of:

adhesive bonding low melting point metal balls each being sphericallyformed and having a given size to the respective electrodes, and

reflowing the low melting point metal balls.

The low melting point metal balls are preferably adhesive bonded to therespective electrodes with a flux.

The flux is preferably applied to the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a semiconductor device of thepresent invention.

FIG. 2 is a view illustrating an electrode in the semiconductor deviceof the present invention.

FIG. 3 is a view illustrating a semispherical bump of the semiconductordevice of the present invention which bump is formed by reflowing a lowmelting point metal ball.

FIGS. 4A to 4D are views illustrating a process for producing thesemiconductor device of the present invention.

FIG. 5 is a view illustrating a bump of a solder ball directly formed ona chip electrode of a single material.

FIGS. 6A and 6B are views illustrating a preferred method for adhesivebonding low melting point metal balls to the respective electrodes.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a semiconductor device 1 of the present invention. Thesemiconductor device 1 is provided with low melting point metal ballbumps 3 adhesive bonded onto the electrodes (not shown) formed on asurface of a semiconductor chip 2.

The low melting metal balls 3 can be formed from one of the varioussolders used for mounting a semiconductor device on a substrate.Examples of the solders include solders of Sn alloys such as a Sn—Pballoy and a Sn—Ag alloy and solders of Pb alloys such as a Pb—In alloy.

The electrodes to which the bumps 3 of low melting point metal balls areadhesive bonded can be formed from an electrode material of Cu or a Cualloy, Al or an Al alloy, or Au or a Au alloy. In the semiconductordevice of the present invention, an electrode having a surface area of900 to 22,500 μm² is preferably used. That is, when a square electrodeis used, one of the sides of the electrode has a dimension of 30 to 150μm.

When the electrode material is Al or an Al alloy, connecting solderballs (the term solder balls designates low melting metal ballshereinafter and is used below) to an electrode by reflowing deterioratesthe bonding between the solder balls and the electrode. When Al or an Alalloy is used as an electrode material, at least one layer of a metal oran alloy of the metal having a melting point higher than the electrodematerial is laminated to the layer formed of the electrode material toavoid the deterioration. A typical example of the material used as thematerial therefor is a metal selected from Ti, W, Ni, Cr, Au, Pd, Cu,Pt, Ag, Sn and Pb, or an alloy of these metals. Of these substances, Ti,W, Ni, Cr, Pd, Cu or Pt, or an alloy of these metals is particularlyeffective in bonding between the material and the layer formed from Alor its alloy. Accordingly, any of the above metals or an alloy of thesemetals is preferably used as a layer contacted with the layer of Al orits alloy. Moreover, a solder generally shows good wettability with Ni,Au, Pd, Cu, Pt, Ag, Sn or Pb, or an alloy of these metals. Of layerslaminated to the electrode material layer of Al or an Al alloy, thelayer contacted with the solder ball is, therefore, preferably formedfrom these materials.

As explained above, when Al or an Al alloy is used, the electrode has amultilayered structure as illustrated in FIG. 2. In FIG. 2, an electrode8 is formed as a laminated structure including a first layer 5 made ofAl (or an Al alloy) on a surface of a semiconductor chip 2, a secondlayer 6 made of Cr on the first layer, and a third layer 7 made of Cu onthe second layer.

In addition to the laminated structure successively having from thesemiconductor chip side an Al (or Al alloy) layer, a Cr layer and a Culayer (such a laminated structure being represented as Al/Cr/Cuhereinafter) as illustrated in FIG. 2, examples of the laminatedstructure of the electrode in which Al or its alloy is used as anelectrode material may include Al/Ni, Al/Ni/Au, Al/Ni/Cu/Au,Al/Cr/Cu/Au, Al/Ti/Cu/Au, Al/Ti/TiW (alloy)/Cu/Au, Al/TiW (alloy)/Cu/Au,Al/Cr/Ni/Pd, Al/Pd/Au, Al/Ni/Sn, Al/Cr/Cu/Pd and Al/Cr/Pt. It isneedless to say that effective electrode laminated structures are notlimited to the structures mentioned above in the semiconductor device ofthe present invention.

A flux is preferably used for adhesive bonding the solder ball to theelectrode. Any of the fluxes generally used in producing semiconductordevices may be used. Although the flux may be applied to either thesolder ball or the electrode surface, it is preferably applied to theelectrode surface. Standard methods such as screen printing may beutilized for the method for applying the flux to the electrode surface.

When the semiconductor device of the present invention is flip chipbonded to a substrate, the solder ball bumps may be made to face therespective electrode terminals of the substrate, positioned, andcontacted therewith, followed by reflowing the bumps. Such a procedureof flip chip bonding has been widely known, and it is needless toexplain in detail.

The semiconductor device of the present invention may be flip chipbonded to the substrate after the solder balls are reflowed once to formsemispherical bumps. FIG. 3 shows an example of a semispherical bump.The semispherical bump 10 in FIG. 3 is formed by reflowing the solderball bump 3 adhesive bonded to the laminated electrode 8 having beenexplained in FIG. 2.

In addition, the bump 10 in FIG. 3 formed by reflowing a solder ball isherein described as semispherical. The term is mainly based on thelongitudinal sectional shape of the bump subsequent to reflowing asshown in FIG. 3. Electrodes on semiconductor chips have various shapessuch as a circular shape, a square shape and other arbitrary shapes. Forexample, a bump formed by reflowing a solder ball on a square electrodehas a semispherical longitudinal sectional shape as seen in FIG. 3.However, since the molten solder wets the entire square electrodesurface and then solidifies, the shape viewed from above (transversesectional shape) is not a circle but a square or a shape close to asquare. Accordingly, it should be noted that the semispherical bumpherein termed includes not only a bump appearing to have a circularcross sectional shape when viewed from above after reflowing but also abump having an arbitrary transverse cross sectional shape reflecting theelectrode shape under the bump. That is, “a semispherical bump” hereindesignates all sorts of bumps formed by reflowing solder balls adhesivebonded to electrodes having an arbitrary shape.

In order to appropriately form a semispherical bump on an electrode byreflowing a solder ball, the radius R of the solder ball to be adhesivebonded to the electrode is desirably selected so that the equation0.4√{square root over (A)}≦R≦2√{square root over (A)}wherein A is the surface area of the electrode, is satisfied. When theradius R of the solder ball is less than 0.4√{square root over (A)}, theamount of the solder becomes insufficient, and formation of a goodsemispherical bump subsequent to reflowing becomes difficult. When theradius R of the solder ball exceeds 2√{square root over (A)}, thesemispherical bump becomes large compared with the size of theelectrode. Consequently, the bonded portion between the electrode andthe bump is subjected to stress concentration, and tends to befractured.

When an electrode having a surface area of 900 to 22,500 m² is used inthe semiconductor device of the present invention, a preferred radius Rof the solder ball derived from the above equation is from 12 to 300 μm.

An example of the production of a semiconductor device in the presentinvention will be explained by making reference to FIG. 4.

As shown in FIG. 4A, a 100×100 μm electrode 42 1.0 μm thick of an Alalloy (Al—Si—Cu alloy) is formed on a semiconductor chip 41 bysputtering. The reference numeral 43 in the figure designates apassivation film which compartments the electrode thus formed. Next, ametal layer 44 of Ni and a metal layer 45 of Cu each having a thicknessof 80 nm are successively laminated to the chip electrode 42 bysputtering, as shown in FIG. 4B.

What is explained above is about the step for forming a substrate onwhich a low melting point metal bump is to be formed. Next, as shown inFIG. 4C, a solder ball 46 of Pb—Sn alloy having a diameter of 80 μm isadhesive bonded to the Cu metal layer 45. At the time of adhesivebonding the solder ball, the surface of the metal layer 45 is firstcoated with a flux (not shown) by screen printing. In cases where, forexample, the solder ball placed on the electrode is subsequently to bereflowed in a reducing atmosphere, coating the metal layer 45 with theflux may be omitted. The solder ball 46 is then adhesive bonded to theflux. A preferred method for adhesive bonding the solder ball to theelectrode will be explained later.

The semiconductor device of the present invention thus prepared can beflip chip bonded to a substrate by making the solder ball bumps face therespective corresponding electrode terminals of the substrate,positioning the bumps, contacting the bumps with the respectiveelectrode terminals thereof, and reflowing the bumps.

The semiconductor device of the present invention provided with thebumps of solder balls 46 as shown in FIG. 4C may also be flip chipbonded to the substrate after forming semispherical bumps 47 byreflowing the solder balls once as shown in FIG. 4D.

Since an Al alloy is used as an electrode material in the above example,the Ni layer and the Cu layer are laminated to the Al alloy layer so asto firmly bond the semiconductor device to the substrate by the solderbump. However, when the electrode material is neither Al nor Al alloy,for example, when the electrode material is Cu or a Cu alloy, or Au or aAu alloy, the substrate (one or more metal (or alloy) layers on theelectrode material layer) for forming low melting point metal bumps isnot required to be formed. Accordingly, as shown in FIG. 5, a bump of asolder ball 53 can be directly formed on an electrode 52 of asemiconductor chip 51. The solder ball bump may also be reflowed once tobe formed into a semispherical bump, which is thereafter used for flipchip bonding.

Another example will now be described, in which solder ball bumps havinga diameter of 150 μm are formed on an electrode having a diameter of 50μm. In this case, Cr, Cu, and Au layers are successively superposed bysputtering process on an electrode of Al—Cu alloy having a diameter of50 μm and a thickness of 1.0 μm, the superposed Cr, Cu, and Au layershaving a thickness of 80 nm, 80 nm, and 30 nm, respectively, and adiameter which is the same as or somewhat larger than the diameter ofthe electrode. The surface of the Au layer is then coated with a flux,on which a solder ball of Pb—Sn alloy having a diameter of 150 μm isadhesive bonded. Shear test carried out for semispherical bumps formedby reflowing the solder balls revealed that all fractures occurred inthe solder balls, and no fracture was observed at the bonded portionsbetween the bumps and electrodes.

Next, a preferred method for adhesive bonding the solder balls to theelectrodes will be explained. At present, an explanation will be made ofadhesive bonding the solder balls to not electrodes each having amultilayered structure but electrodes each composed of a single materialas explained in FIG. 5.

As shown in FIG. 6A, a vibration at a small amplitude is applied to avessel 60 containing the solder balls 53 to cause the solder balls 53 tojump up. The solder balls 53 are then arranged and held on anarrangement base plate 63 by attracting the jumping up solder balls 53to attraction openings 61 (attracting mechanism for attracting thesolder balls being not shown) provided in the arrangement base plate 63in positions corresponding to positions of the electrodes of thesemiconductor chip to which the solder balls 53 are to be adhesivebonded. During attracting and arranging the solder balls, excess solderballs 53′ adhere to portions of the arrangement base plate 63 other thanthe attraction openings 61, or other excess solder balls 53″ adhere tothe solder balls 53 attracted to the attraction openings 61, as shown inFIG. 6A. The excess solder balls 53′, 53″ are, therefore, removed. Toachieve the removal, arbitrary procedures may be utilized. For example,excess solder balls 53′, 53″ can be preferably removed by applying anultrasonic vibration to the arrangement base plate 63 in the horizontaldirection. Although only two attraction openings 61 are shown in theattraction base plate 63 in FIG. 6A for the sake of simplicity, itshould be noted that the actual arrangement base plate has theattraction openings the number of which is the same as that of thesolder balls to be adhesive bonded to the electrodes of thesemiconductor chip.

Next, as shown in FIG. 6B, the arrangement base plate 63 holding thesolder balls 53 in predetermined positions is moved above thesemiconductor chip 51 so that the solder balls 53 are properlypositioned with respect to the respective electrodes 52 of thesemiconductor chip 51.

The arrangement base plate 63 is then moved downward so that the solderballs 53 are contacted with the respective electrodes 52. After thecontact, attracting the solder balls 53 to the arrangement base plate 63is stopped (by stopping the attraction mechanism), and the arrangementbase plate 63 is moved upward.

When the surface of the electrodes 52 is coated with a flux (not shownin the figure), the solder balls 53 are adhesive bonded to theelectrodes due to the adhesion thereof. When the surface of theelectrodes 52 is not coated with a flux, semispherical bumps adhesivebonded to the respective electrodes 52 can be formed by, for example,reflowing the solder balls 53 in a reducing atmosphere, as referred toabove.

Since solder bumps can simultaneously be adhesive bonded to a largenumber of the respective electrodes of a semiconductor chip by theprocess as explained above, the process is very advantageous to theproduction of the semiconductor device of the present invention.

In general, a large number of semiconductor chips are formed on onewafer, and separated by cutting to give individual chips. The process asmentioned above may also be applied to a plurality of semiconductorchips prior to separation of them from the wafer by cutting, or it maybe applied to individual semiconductor chips subsequently to separationof them therefrom. It is evident from what has been explained above thatthe semiconductor chip in the present invention includes not only aseparated individual semiconductor chip but also a plurality ofsemiconductor chips in a state of being produced on one wafer.

It is evident from what has been explained above that the semiconductordevice of the present invention is provided with low melting point metalball bumps directly adhesive bonded to the respective electrodes formedon a semiconductor chip. The bumps can be made of high quality by makingthe size of the metal balls uniform. The metal balls are not formed onthe electrodes of the semiconductor chip by a procedure such as platingor vapor deposition in conventional processes, but can be adhesivebonded to the electrodes. Accordingly, the semiconductor device of thepresent invention can be produced without a mask, and without fear ofenvironmental pollution.

Furthermore, the amount of bumps can be easily and highly accuratelycontrolled by adjusting the size of the low melting point metal balls,to enhance reliability of bumps.

INDUSTRIAL APPLICABILITY

The present invention can be advantageously applied to flip chip bondingwhich makes possible high density mounting of a semiconductor device ona substrate.

1. A process for producing a semiconductor device to be mounted on asubstrate by flip chip bonding comprising electrodes formed on asemiconductor chip, and spherically formed metal balls having a diameterof not greater than 150 micrometers, and adhesive bonded to theelectrodes (8), wherein each electrode (8) includes a layer of anelectrode material (5) and at least one layer (6, 7) laminated to thelayer of the electrode material (5) to avoid deterioration of bondingsuch that the at least one layer (6, 7) has peripheral dimensionssubstantially the same as or larger than those of the electrode material(5); said process comprising: adhesive bonding the metal balls to theelectrodes with a flux without reflowing the metal balls, whereby themetal balls are only bonded to the electrodes with flux withoutreflowing, and are physically attached to tops of the electrodes.
 2. Aprocess for producing a semiconductor device to be mounted on asubstrate by flip chip bonding comprising electrodes formed on asemiconductor chip, and spherically formed metal balls having a diameterof not greater than 150 micrometers, and adhesive bonded to theelectrodes (8), wherein each electrode (8) includes a layer of anelectrode material (5) and at least one layer (6, 7) laminated to thelayer of the electrode material (5) to avoid deterioration of bondingsuch that at least one of the at least one layer (6, 7) has a thicknesswhich is smaller than that of the electrode material (5) and the atleast one layer (6, 7) has peripheral dimensions substantially the sameas or larger than those of the electrode material (5); said processcomprising: adhesive bonding the metal balls to the electrodes with aflux without reflowing the metal balls, whereby the metal balls are onlybonded to the electrodes with flux without reflowing, and are physicallyattached to tons of the electrodes.
 3. The process according to claim 1or 2 comprising: applying the flux to the electrodes.
 4. The processaccording to claim 1 or 2, wherein the metal balls are adhesive bondedto the electrodes by a process comprising the steps of: applying avibration at a small amplitude to a vessel containing the metal balls tocause the metal balls to jump up; arranging and holding the metal ballson an arrangement base plate by attracting the jumping up metal balls toattraction openings provided in the arrangement base plate in positionscorresponding to the electrodes of the semiconductor chip to which themetal balls are to be adhesive bonded; removing excess metal ballsadhering either to the arrangement base plate or to the metal ballsattracted to the attraction openings; and simultaneously contacting themetal balls held and arranged on the arrangement base plate with theelectrodes of the semiconductor chip.
 5. A semiconductor device producedby the process of claim 1 or 2, provided with metal balls adhesivebonded to the respective electrodes of the semiconductor chip.